Log message:
* If PLIST_SRC is explicitly set to an empty value in a package Makefile,
then automatically generate a PLIST that says "${PKGNAME} has no files".
* If PLIST_SRC and GENERATE_PLIST are not set in a package Makefile,
and no PLIST files exist, then fail during the package build with
PKG_FAIL_REASON.
* Remove "intentionally empty" PLISTs again.
Now, the easy way to say that a package installs no files is to just
add the following to the package Makefile:
PLIST_SRC= # empty

Log message:
Re-add "intentionally empty" PLISTs for meta-packages and other packages
that directly manipulate empty PLISTs.
Modify plist/plist.mk so that if the PLIST files are missing and no
GENERATE_PLIST is defined, then the package fails to build.